At first glance, routers seem a lot like a PC. They have a CPU; memory; and, on the back, ports and interfaces to hook up peripherals and various communications media. They sometimes even have a monitor to serve as a system console.

But there's one defining difference from a PC: routers are diskless. They don't even have floppy disks. If you think about it, this makes sense. A router exists to do just that: route. They don't exist to create or display the data they transport. Routers have as their sole mission the task of filtering incoming packets and routing them outbound to their proper destinations.

Another difference is in the kind of add-on modules that can be plugged into routers. Whereas the typical PC contains cards for video, sound, graphics, or other purposes, the modules put into routers are strictly for networking (for obvious reasons). These are called interface modules, or just plain interfaces. When people or documents refer to a router interface, they mean an actual, physical printed circuit board that handles a particular networking protocol. E0 and E1, for example, probably mean Ethernet interface numbers 1 and 2 inside a router. There is one protocol per interface.

Interfaces are added according to the network environment in which they will work. For example, a router might be configured with interface modules only for Ethernet. A router serving in a mixed-LAN environment, by contrast, would have interfaces for both the Ethernet and Token Ring protocols, and if that router were acting as a LAN-to-WAN juncture, it might also have an ISDN module.

Router Memory

Routers use various kinds of memory to operate and manage themselves. There are many variations on what the internals of a router look like (based on what they do and how they do it). Figure 3-16 depicts the layout of a motherboard in a Cisco router. This is a classic "old school" router with a good layout for identifying the various parts of a router. All Cisco router motherboards use four types of memory, each dedicated to performing specific roles.

image from book
Figure 3-16: Motherboards of Cisco routers are similarly designed

Each Cisco router ships with at least a factory default minimum amount of DRAM and flash memory. Memory can be added at the factory or upgraded in the field. As a general rule, the amount of DRAM can be doubled or quadrupled (depending on the specific model), and the amount of flash can be doubled. If traffic loads increase over time, DRAM can be upgraded to increase a router's throughput capacity.


RAM/DRAM stands for random access memory/dynamic random access memory. Also called working storage, RAM/DRAM is used by the router's central processor to do its work, much like the memory in your PC. When a router is in operation, its RAM/DRAM contains an image of the Cisco IOS software, the running configuration file, the routing table, other tables (built by the router after it starts up), and the packet buffer.

Don't be thrown by the two parts in RAM/DRAM. The acronym is a catch-all. Virtually all RAM/DRAM in Cisco routers is DRAM-dynamic random access memory. Nondynamic memory, also called static memory, became obsolete years ago. But the term RAM is still so widely used that it's included in the literature to avoid confusion on the subject.

Cisco's smallest router, the SOHO 90 Series, ships with a maximum of 64 MB of DRAM. At the other end of the spectrum, the CRS-1 router, one of Cisco's largest, supports up to 4 GB.


NVRAM stands for nonvolatile RAM. Nonvolatile means memory that will retain information after losing power. Cisco routers store a copy of the router's configuration file in NVRAM (configuration files are covered later in this chapter). When the router is intentionally turned off, or if power is lost, NVRAM enables the router to restart in its proper configuration.

Flash Memory

Flash memory is also nonvolatile. It differs from NVRAM in that it can be erased and reprogrammed as needed. Originally developed by Intel, flash memory is in wide use in computers and other devices. In Cisco routers, flash memory is used to store one or more copies of the IOS software. This is an important feature, because it enables network managers to stage new versions of IOS on routers throughout an internetwork and then upgrade them all at once to a new version from flash memory.


ROM stands for read-only memory. It, too, is nonvolatile. Cisco routers use ROM to hold a so-called bootstrap program, which is a file that can be used to boot to a minimum configuration state after a catastrophe. When you boot from ROM, the first thing you'll see is the rommon>> prompt. ROMMON (for ROM monitor) harks back to the early days of the UNIX operating system, which relied on ROMMON to reboot a computer to the point at which commands could at least be typed into the system console monitor. In smaller Cisco routers, ROM holds a bare-bones subset of the Cisco IOS software. ROM in some high-end Cisco routers holds a full copy of IOS.

Router Ports and Modules

A router's window to the internetwork is through its ports and modules. Without them, a router is a useless box. The ports and modules that are put into a router define what it can do.

Internetworking can be intimidating, with the seemingly endless combinations of products, protocols, media, feature sets, standards-you name it. The acronyms come so fast and so hard that it might seem hopeless to learn how to properly configure a router. But choosing the right router product can be boiled down to manageable proportions. Table 3-4 lays out five major requirement areas that, if met, will lead you to the best router solution.

Table 3-4: Five Major Factors in Selecting a Router



Configuration Requirement



It must be hardware-compatible with the physical network segment on which the router will sit.



The router must be compatible with the transport medium that will be used (Frame Relay, ATM, etc.).



It must be compatible with the protocols used in the internetwork (IP, IPX, SNA, etc.).



The router must provide the speed, reliability, security, and functional features the job requires.



It must fit within the purchase budget and network growth plans.

Cisco obviously can't manufacture a model of router to match every customer's specific requirements. To make them more flexible to configure, routers come in two major parts:

  • Chassis The actual box and basic components inside it, such as power supply, fans, rear and front faceplates, indicator lights, and slots

  • Ports and modules The printed circuit boards that slide into the router box

Cisco's router product-line structure tries to steer you to a product-or at least to a reasonably focused selection of products-meeting all five requirement areas in Table 3-4.

Finding the right router for your needs is basically a three-step process. The following illustrates the process of selecting a router for a large branch office operation:

image from book

First, Cisco's routers are grouped into product families called series. Choosing a router product series is usually a matter of budget, because each series reflects a price/performance tier. Models within series are generally based on the same chassis, which is the metal frame and basic components (power supply, fans, and so on) around which the router is built. We'll select the Cisco 2000 Series because it fits both the purchase budget and performance requirements for our large branch office.

image from book

From the 2000 Series, we'll take the Cisco 2800 Series. The 2800 chassis is versatile enough to fit a lot of situations, making it a popular brand of branch office router.

image from book

Third, we'll select the Cisco 2851 because it has two Ethernet ports; and our imaginary branch office will operate two subnets, one for the customer service office and another for the front office. The two Ethernet ports will let us separate the two departments, thereby isolating traffic.


The term "port" can cause confusion if you're not careful. When speaking of hardware, port means a physical connection through which I/O can pass (a serial port, for example), but there are also so-called ports at the transport layer of network protocols. These "ports" are actually port numbers used to identify what network application packets contain. These ports (port numbers) are also referred to as TCP or UDP ports or "listeners," because they inform the receiver what's inside the message. Example TCP-defined port numbers include Port 25 for Simple Mail Transfer Protocol and Port 80 for HTTP. Refer to the section in Chapter 2, "The Transport Layer," for more on TCP and UDP ports.

Router Packaging

Three major categories of modules can be configured into Cisco routers to support either LAN or WAN connectivity:

  • Ethernet modules Support many of the Ethernet LAN variants on the market, including Novell NetWare, Banyan VINES, and AppleTalk.

  • Token Ring modules IBM's LAN technology, which is well established in banks, insurance companies, and other Fortune 1000 corporate environments.

  • WAN connectivity modules Support a wide variety of WAN protocols, some old and some new. Example WAN technologies include newer protocols, such as ISDN, Frame Relay, Asynchronous Transfer Mode (ATM), as well as legacy protocols, such as SDLC and X.25.

Configuration options depend mainly on the specific Cisco router:

  • Lower-end routers tend to be "fixed configuration" in that the modules are factory integrated only (preconfigured).

  • Midrange routers, such as the Cisco 3800 Series, are "modular" in that they can accept a variety of modules, often packaging different protocols in the same box. Interface modules are plugged into this class of routers' motherboards.

  • High-end routers, such as the Cisco 7300 Series and Cisco 12000 Series, have buses (also called backplanes). Bus-based routers accept larger modules-usually referred to as blades or cards-that are effectively self-contained routers (they have their own CPUs, memory units, and so on).

Cisco Router Models

Cisco offers different routers for different environments. For example, an ISP won't be too successful with a router that can only accommodate four or five users. Likewise, a small office doesn't need a $500,000 terabit router. The following explain the different types of routers Cisco offers.

SOHO Routers

SOHO is an industry term for very small network users-small office/home office. Typical SOHO customers have only one or two LAN segments in their facility and an ISP connection to the Internet.

The linchpin of Cisco's SOHO strategy is low-end routers. Allowing small companies to tap into ISPs router-to-router instead of as dial-in users saves money on telephone connections, upgrades performance, and improves reliability. Table 3-5 outlines Cisco's SOHO product series. The term series here means a chassis that is variously configured at the manufacturing plant into several product models-usually depending on the printed circuit cards installed in them.

Table 3-5: Cisco's Three Series of SOHO Routers

Product Series


Cisco 90 Series

DSL access router to connect multiple users. Includes user-configurable IOS.

Cisco 800 Series

ISDN, DSL, and serial access routers to connect up to 20 users. Includes IOS and has VPN encryption capability.

Cisco SB 100 Series

Aimed specifically at small businesses, this series of Ethernet, ISDN, and DSL routers include userconfigurable IOS.

SOHO products emphasize broadband technologies (DSL, cable, and ISDN) because small offices and home offices don't have dedicated WAN links connecting them to their ISP or enterprise internetwork.

Midrange Cisco Routers

The smallto medium-sized network requires a wide variety of solutions. Cisco has several tiers of access router products designed to fit the customer's capacity needs and type of telecom link.

The series in Table 3-6 represent dozens of individual product numbers. Depending on the product series, various combinations of LAN technologies and WAN media can be configured. Modular means that the chassis can be upgraded in the field by inserting one or more modules. Nonmodular devices are fixed in configuration.

Table 3-6: Cisco's Midrange Routing Solutions

Product Series


Cisco 1700 Series

Ethernet access router to connect to a broadband, ISDN, or DSL WAN link. VPN encryption, VoIP (voice over IP), and VoFR (voice over Frame Relay) capability.

Cisco 1800 Series

Ethernet access router to connect to ISDN, DSL, broadband. High-density router, offers support for 802.11 a/b/g technologies, multiple WAN interface options, and VPN capability.

Cisco 2600 Series

Integrated, modular, and cost-effective solution for duty as a multiservice access router, voice/data gateway, or dial-access server. Connects one or two Ethernet or Token Ring LANs to ISDN, channelized T1, Ethernet, analog modems, or ATM links. Also supports voice, video, FAX, and Frame Relay.

Cisco 2800 Series

Integrated, modular access router. Supports up to two Ethernet LAN segments, wireless (802.11 a/b/g) LANs, along with Cisco IP phones.

Cisco 3600 Series

Modular high-density router for dial-access or router-to-router traffic. Supports ISDN, serial, channelized T1, digital modems, and ATM links. Also supports voice/fax and Frame Relay.

Cisco 3700 Series

A lynchpin in Cisco's AVVID technology, this series of multiservice access routers offers support for major WAN protocols and includes two LAN ports and three WAN slots.

Cisco 3800 Series

Geared toward medium to large businesses and enterprise branch offices, this model operates at up to T3 speeds, and is capable of data, security, voice, video, and wireless functions. This series is modular and offers VPN capabilities.

Backbone Routers

When Cisco claims that over 70 percent of the Internet is run using its routers, these are the models they're talking about. The 7000 and 12000 series are big, resembling dorm refrigerators in shape and size, and have data buses into which blades (whole devices on a board) can be installed.

Because they're not access routers, Cisco's backbone routers can take as many users as they can handle packets. All seven product series (described in Table 3-7) are modular, letting customers install modules according to the LAN technology being run and the capacity needed. In fact, these routers can operate more than one protocol simultaneously, such as Ethernet and Token Ring. A slot is an electronic bay into which a printed circuit board module is inserted.

Table 3-7: Cisco's Series of Backbone Routers

Product Series


Cisco 7200 Series

Four or six slots. Multiprotocol support, scalable, and flexible. Supports Fast Ethernet, Gigabit Ethernet, and Packet Over SONET. Offers MPLS support and OC-12/STM-4 rates.

Cisco 7300 Series

Oneor four-slot router marketed to campus WAN or service providers. Model includes firewall, networkbased application recognition, QoS (Quality of Service), IP/MPLS services, and data rates of up to OC-48/STM-16.

Cisco 7500 Series

Sevenor 13-slot router offering distributed services capabilities and multiprotocol support. Including distributed switching and high port density, this series boasts high availability. Offers data rates up to OC-12/STM-4 speeds.

Cisco 7600 Series

Three to 13 slots with 240to 720-Gbps data rate per slot. This series offers a flexible, modular design, IP/MPLS services, and is an easy upgrade from the Cisco 7500 Series. Offers up to OC-48/STM-16 speeds.

Cisco 10700

Two-slot router offering differentiated IP services at optical speeds of up to OC-48/STM-16.

Cisco 10000 Series

Eight-slot Gigabit Ethernet switch router. Has special card for OC-48 WAN link. Offers IPSec and MPLS VPN and QoS capabilities.

Cisco 12000 Series

Six-or 15-slot, 40-Gigabit per-slot Ethernet switch router optimized for IP. Has special cards for OC-48 and OC-192 WAN links.

Cisco CRS-1 Series

Eight to 16 slots, powered by Cisco IOS XR software, which offers a self-healing, self-defending operating system. Supports fixed and modular line cards. The kingpin of Cisco's router line, it offers terabit speeds with OC-768 support.

There is a specialized high-speed connection used on the high-end Cisco routers. HSSI stands for high-speed serial interface, a specialized I/O standard mainly used in conjunction with supercomputers. The behemoth Cisco 12000 router and uBR10012 router are carrier-class devices, in that local equipment carriers' telecommunications network operators use them in their back-office data-switching operations.

All the backbone Cisco routers have extensive capabilities for VPN, security, quality of service (QoS), and network management.

Cisco. A Beginner's Guide
Cisco: A Beginners Guide, Fourth Edition
ISBN: 0072263830
EAN: 2147483647
Year: 2006
Pages: 102

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